In conventional coating apparatus of the kind described above, the thicknesser blade is strengthened mechanically by being secured along its entire length to a blade support. The connection between the blade and the blade support is along that portion of the blade support which is closest to the web plane. The blade needs to be strengthened mechanically in order to minimise bending of the blade in the blade plane, as a result of mechanical stresses during operation. It will be appreciated that bending of the blade in the blade plane would cause a variation in the width of the thicknesser gap and this is not wanted. However, the construction referred to above does lead to certain difficulties. First, the blade needs to be machined extremely accurately throughout its length in order to ensure a uniform gap width. Second, if the blade is used in a so-called hot knife coater, wherein the coating material is hot, that portion of the blade immediately adjacent the thicknesser gap heats up to about the temperature of the coating material, whereas that portion of the blade support remote from the blade remains at about room temperature. The temperature differential thus set up across the width of the blade assembly (i.e. the assembly of blade and blade support) causes the blade assembly to bend or bow in the blade plane despite the mechanical strength of the assembly in this plane. Even slight bending (e.g. with the blade having a radius of curvature in the order of 1 to 2 km) can cause defects in the material produced by the apparatus. The problem is aggravated by the fact that the coating material is usually applied to the web by passing the web through a bath of the material and that the bath is replenished with hot coating material at intermittent intervals and often at one point along the length of the bath. Thus, not only does the temperature of the blade vary along the length of the blade, it also fluctuates in time.
Another difficulty experienced with conventional coating apparatus of the kind described above is that when a seamed portion of the web is to pass through the thicknesser gap, the gap has to be opened up by operator intervention. Failure by the operator to take the necessary action can result in damage to large portions of the material produced by the apparatus.
It is an object of the present invention to overcome or at least alleviate the above difficulties.